Lensless and Optical Physically Unclonable Function with Fibrous Media

Abstract

Physically unclonable functions (PUFs) are physical security systems cannot be easily replicated because the manufacturing process causes inherently uncontrollable random factor in same elements. The manufactured PUF acts as unique as a fingerprint. As a result, it is nearly impossible to replicate a sufficiently identical PUF. Therefore, PUF is a promising and reliable solution in hardware security, such as secure authentication, identification, and encryption key generation. The concept of PUF was first proposed as physical one-way functions in 2001 by R.S. Pappu [1]. Since then, various types of PUFs have been suggested, including optical-based PUF, magnetic-based PUF, electronics-based PUF, and RF-based PUF [2]. In particular, the optical PUF provides numerous challenge and response pair. Fig. 1(a) illustrates the configuration of self-focusing effect in fibrous media. The fibrous media can present random pinhole pattern according to fibers density. Fig. 1(b) illustrates a single stochastic pinhole composed of fibers. The propagated light is diffracted by the fibers, and then the diffracted light can be focused by constructive interference. Thus, the proper size of pinholes can cuase focal spots by the diffraction at far-field distance under visible lights as shown in Fig. 1(c). Due to such ‘self-focusing effect’, challenge and response pair are generated without additional imaging optics in fibrous media-based PUF. Fig. 1(d) illustrates the configuration of lensless, optical, and portable PUF (LOP-PUF). A fibrous material, silk, is directly placed on chromatic complementary metal-oxide-semiconductor (CMOS) image sensor, and the three tri-color light emitting diodes (LEDs) illuminate different wavelengths (i.e., blue, green, and red) at three angles (i.e., 0º, -15º, and +15º). Each tri-color LED selectively emits red, green, and blue lights by changing anode and cathode. To expose the illuminated lights on the silk, a silver mirror is slanted, and the mirror obliquely reflects the incident light to the PUF-tag. To deeply investigate the structural effects for lensless imaging, three different density fibrous ID cards (i.e., porous polymer, paper, and silk) are fabricated. All ID cards are highly scattering tags, which display white surfaces. Porous polymer and paper sample offer a dull blue surface without any remarkable focal spots, whereas only silk with optimum fiber density shows notable focal spots (Fig. 1(e) and Fig. 1(f)).